Our overall goals are to determine how a simple vertebrate auditory system functions in the fundamental processes of hearing: the detection, identification, classification, and the location of sound sources. During the last grant period we demonstrated that goldfish (Carassius auratus) behave as if they had internal perceptual dimensions corresponding to spectral pitch, virtual pitch, timbre, and roughness. In addition, physiological experiments have revealed that the simple, discrete-channel frequency selectivity of the periphery is transformed at or prior to the level of the midbrain into more sharply-tuned and continuously distributed filterbank array that has been synthesized by inhibition. Inhibition in the midbrain has also synthesized novel directional response patterns. These and other results have led to the hypothesis that the physiological functions of the mammalian auditory system and the sense of hearing among mammals as revealed in psychoacoustical studies are primitive vertebrate characters shared by fishes and other non-mammalian taxa. At the same time, fishes represent an extreme position among vertebrates with respect to phylogeny, inner ear structure, and peripheral representations of frequency. Understanding the mechanisms of hearing among fishes is thus an important bench-mark for further evaluating the dimensions of variation and similarity among vertebrates as a group, and for establishing a biological context within which human hearing can be more completely understood. This project focuses on two fundamental aspects of the vertebrate sense of hearing: the processing to spectrally and temporally complex sounds evoking perceptions of pitch, and the abilities and mechanisms for sound source localizations. The complementary behavioral, neurophysiological, and modeling studies proposed will help identify the neural structures, codes, and mechanisms underlying these fundamental aspects of vertebrate hearing.